Fermented black soldier fly larvae as a sustainable replacement for marine fish in Asian swamp eel diets.

Background and Aim: Fermented black soldier fly larvae (BSFL) have emerged as a sustainable and economically viable protein source in aquaculture. However, their potential as a replacement for marine fish in the diets of Asian swamp eels ( Materials and Methods: A total of 480 ASEs were randomly assigned to four dietary groups: control (40% marine fish), BSFL34 (13.4% BSFL), BSFL61 (24.1% BSFL), and BSFL82 (32.8% BSFL), replacing marine fish on a dry matter basis. All diets were isonitrogenous and isoenergetic. Fish were reared in net cages for over 90 days, and parameters including survival rate, growth metrics, muscle and liver histology, intestinal morphology, gene expression (quantitative real-time polymerase chain reaction), and inflammatory protein levels (Western blotting) were assessed. Results: Survival rate was significantly higher in the BSFL61 group (p < 0.05). Growth performance was not impaired across BSFL-fed groups, although BSFL61 showed reduced body weight compared to BSFL82 (p < 0.05). Muscle fiber size, satellite cell number, and muscle triglyceride (TG) content remained unchanged. BSFL82 showed increased hepatic TG accumulation (p < 0.05) and reduced liver fibrosis, while BSFL61 exhibited a significantly lower hepatosomatic index and increased fibrosis. Intestinal villus height was reduced in BSFL34 and BSFL61, while goblet cell density increased in all BSFL groups. Notch1 expression was upregulated in BSFL61 and BSFL82, whereas ctnnb1 and wnt5a were downregulated. Inflammatory markers nuclear factor-kappa B and interleukin-1 beta were elevated in BSFL-fed groups, indicating an activated mucosal immune response. Conclusion: Partial replacement of marine fish with fermented BSFL enhanced ASE survival, modulated intestinal immunity, and improved mucosal barrier function, without compromising overall growth performance. However, excessive inclusion may induce hepatic lipid accumulation and affect intestinal morphology. These findings support the use of fermented BSFL as a sustainable aquafeed ingredient, though inclusion levels should be carefully optimized to balance health benefits and growth efficiency.